III-V group nitride semiconductors which contain nitrogen (N) as their V group element are regarded as promising materials for short-wavelength light-emitting devices, due to their large band gaps. Among others, vigorous researches are being directed toward gallium nitride-type compound semiconductors (GaN-type semiconductors: AlGaInN), and blue light-emitting diodes (LEDs) and green LEDs have already been put to practical use. Moreover, for the sake of realizing large-capacity optical disk apparatuses, semiconductor lasers having an oscillation wavelength in the 400 nm band are being aspired for. Semiconductor lasers using a GaN-type semiconductor as their material are attracting attention, and are currently coming up to practical levels.
GaN-type semiconductor lasers are disclosed in, for example, Japanese Laid-Open Patent Publication No. 10-126006; Japanese Journal of Applied Physics, Vol. 38, L226-L229 (1999); physica status solidi (a) 194, No. 2, 407-413 (2002); and the like.
Hereinafter, with reference to FIG. 1 and FIG. 2, conventional GaN-type semiconductor lasers will be described.
First, FIG. 1(a) will be referred to.
A semiconductor laser shown in FIG. 1(a) includes a low-dislocation ELO-GaN substrate 101, and a multilayer structure of nitride semiconductors epitaxially grown on the ELO-GaN substrate 101. The ELO-GaN substrate 101 is composed of a thick GaN film which is produced through lateral epitaxial overgrowth.
Beginning from the substrate 101, the semiconductor multilayer structure of FIG. 1(a) includes: an n-Al0.015Ga0.985N contact layer 102; a Ga0.95In0.05N crack suppression layer 103; an n-Al0.15Ga0.85N/n-GaN superlattice (SLs) cladding layer 104; a GaN optical guide layer 105; a Ga0.86In0.14N/Ga0.99In0.01N multiple quantum well (MQW) active layer 106; a p-Al0.20Ga0.80N capping layer 107; a GaN optical guide layer 108; a p-Al0.15Ga0.85N/p-GaN-SLs cladding layer 109; and a p-GaN contact layer 110. Crystal growth of these semiconductor layers is performed by using metal-organic vapor phase epitaxy (MOVPE technique), for example.
The semiconductor multilayer structure constructed as above is processed into a shape as shown in FIG. 1(a), with a p electrode 111, an SiO2 layer 112, and an n electrode 113 being formed thereon.
FIG. 1(b) schematically shows a conduction-band structure of this semiconductor laser. The horizontal axis of FIG. 1(b) corresponds to the distance from the substrate surface; the more toward the left side in the figure, the farther away from the substrate surface. The vertical axis is the energy level at the lower end of the conduction band.
A feature of this semiconductor laser is that, in order to suppress evaporation of In from the active layer during crystal growth and suppress overflowing of electrons from the active layer, the p-Al0.20Ga0.80N capping layer 107 having the largest forbidden band width is formed immediately above the MQW active layer 106.
Next, another GaN semiconductor laser will be described with reference to FIG. 2(a).
Similarly to the semiconductor laser of FIG. 1(a), this semiconductor laser includes an ELO-GaN substrate 201 and a semiconductor multilayer structure formed thereon. The semiconductor multilayer structure includes an n-GaN contact layer 202, an n-Al0.08Ga0.92N cladding layer 203, a GaN optical guide layer 204, a Ga0.90In0.20N/Ga0.98In0.02N-MQW active layer 205, a GaInN intermediate layer 206, an AlGaN intermediate layer 207, a p-Al0.26Ga0.84N electron blocking layer 208, a p-Al0.15Ga0.85N/p-GaN-SLs cladding layer 209, and a p-GaN contact layer 210.
This semiconductor multilayer structure is processed into a shape as shown in FIG. 2(a), with a p electrode 211, an SiO2 layer 212, and an n electrode 213 being formed thereon.
A schematic diagram of a conduction-band structure of this semiconductor laser is shown in FIG. 2(b). A feature of this semiconductor laser is that, in order to minimize light absorption losses associated with the layers doped with p-type impurities, spacer layers called the GaInN intermediate layer 206 and the AlGaN intermediate layer 207 are formed between the MQW active layer 205 and the p-Al0.26Ga0.84N electron blocking layer 208. Moreover, since the GaInN intermediate layer 206 and the AlGaN intermediate layer 207 function also as optical guide layers, no p-type optical guide layers are formed, unlike in the semiconductor laser shown in FIG. 1(a).
Both GaN semiconductor lasers structured as above have been reported to attain continuous oscillation with an output of 30 mW at room temperature.
However, in the semiconductor laser having the structure as shown in FIGS. 1(a) and (b), the active layer 106 having the largest lattice constant in the laser structure is in contact with the AlGaN capping layer 104 having the smallest lattice constant. Therefore, large strain is imposed on the active layer 106, so that the uniformity and reproducibility during the fabrication of the laser device are not very good.
On the other hand, in the semiconductor laser having the structure as shown in FIGS. 2(a) and (b), intermediate layers are inserted between the active layer 205 and the AlGaN electron blocking layer (which serves the same function as that of the AlGaN capping layer 107 of FIG. 1). Therefore, the strain imposed on the active layer is effectively alleviated, thus resulting in an improved uniformity at fabrication of the laser device.
What is common between these semiconductor lasers is that doping of acceptor impurities for realizing p-type crystals is begun when performing the growth of an AlGaN capping layer (AlGaN electron blocking layer) which has the largest band gap in the laser structure.
Usually in nitride-type semiconductors, magnesium (Mg) is often used as an acceptor impurity. It is known that Mg is associated with a phenomenon called “memory effect” at doping during crystal growth. The “memory effect” occurs due to a time lag from the point at which impurity doping is begun during crystal growth until the impurity which has been introduced to the crystal through doping is actually taken in. More specifically, when this memory effect occurs, the doping start position may be shifted from the intended position toward the crystal growth surface, and the concentration increase of the impurity concentration along the depth direction distribution may become draped rather than steep.
Moreover, conversely to the above situation, the “memory effect” may cause a time lag from the point at which doping is finished during crystal growth until the taking-in of the impurity which is introduced to the crystal through doping actually ends. In this case, the doping end position may be shifted from the intended position toward the surface of the crystal growth layer, and the decrease in the Mg concentration of the impurity concentration along the depth direction distribution may become draped rather than steep.
If this memory effect occurs in the case where the start position of Mg doping is set at the start position of the growth of the AlGaN capping layer (AlGaN electron blocking layer) which has the largest band gap in the laser structure, the AlGaN capping layer (AlGaN electron blocking layer) will have portions whose Mg concentration is locally lowered.
In general, the activation energy of an acceptor impurity tends to increase as the band gap energy increases, i.e., as the Al mole fraction in the crystal increases. If portions having low Mg concentrations are created due to Mg doping lags associated with the memory effect, Mg will not be sufficiently activated in such portions, whereby the function of the AlGaN electron blocking layer will be degraded.
In another conventional example described in Japanese Laid-Open Patent Publication No. 2000-143396, Al doping at 2×1020 cm−3 or less is performed when fabricating a p-GaN crystal (this is not to obtain a mixed crystal of AlGaN because it is only performed to a degree where the band gap energy does not change from that of GaN). It describes that the strain in the crystal associated with Mg doping is consequently alleviated, and the Mg as an acceptor impurity is effectively introduced to the positions of Ga atoms in the GaN crystal, thus obtaining a high hole concentration. However, even if Al doping is performed, the Mg concentration in itself has a constant value. Although the activation rate of the acceptor will be improved and a high hole concentration will be realized by controlling the optimum Al concentration, there is no descriptions concerning suppression of the memory effect at the doping interface, or any acceptor impurity profile having a high controllability and steepness.
In still another conventional example which is described in Japanese Laid-Open Patent Publication No. 2002-198314, in order to improve the steepness of the Mg doping profile, a multi-hetero structure (superlattice structure) of AlGaN layers/GaN layers or GaInN layers/GaN layers is fabricated and doped with Mg. According to this method, even if uniform Mg doping is performed, at the hetero interface between a GaN layer and an AlGaN layer or between a GaN layer and a GaInN layer, the Mg concentration has a local increase near the interface of the AlGaN layer or the GaInN layer that is closer to the substrate (closer to where the growth is begun). By utilizing this phenomenon where the Mg concentration is locally increased, interfaces are abundantly formed in order to improve steepness. However, although this method provides an improved steepness, Mg doping lags still occur in the AlGaN layers (which have a large band gap) so that non-uniformity of Mg concentration exists in the AlGaN layers. Moreover, by forming a multitude of hetero interfaces, excesses and insufficiencies of Mg concentration irregularly occur within the crystal, thus resulting in a very low controllability and reproducibility.
In view of the above, the inventors have fabricated a laser structure as shown in FIG. 3(a). The device of FIG. 3(a) has an ELO-GaN substrate 301. The ELO-GaN substrate 301 is an ABLEG (Air-Bridged Lateral Epitaxial Growth) substrate, and has an air gap structure which is not shown. ABLEG is disclosed in detail in Japanese Laid-Open Patent Publication No. 2002-009004, for example.
A semiconductor multilayer structure on the substrate 301 includes an n-GaN contact layer 302, an n-Al0.07Ga0.93N cladding layer 303, an n-GaN optical guide layer 304, a Ga0.90In0.10N/Ga0.98In0.02N-MQW active layer 305, a GaInN intermediate layer 306, a GaN intermediate layer 307, a p-GaN acceptor impurity doping start layer 308, a p-Al0.16Ga0.84N electron overflow suppression layer 309, a p-Al0.14Ga0.86N/p-GaN-SLs cladding layer 310, and a p-GaN contact layer 311. FIG. 3(b) schematically shows a conduction-band structure of this laser structure.
A feature of this laser structure is that, instead of beginning an acceptor impurity doping concurrently with beginning the growth of the p-Al0.16Ga0.84N electron overflow suppression layer 309 having the largest band gap in the laser structure, the p-GaN acceptor impurity doping start layer 308 is provided before that. Although the p-GaN acceptor impurity doping start layer 308 has a doping lag due to the memory effect, a film thickness designing which takes the doping lag into account makes it possible to keep a constant Mg doping concentration in the p-Al0.16Ga0.84N electron overflow suppression layer 309.
However, in these conventional growing methods, although the Mg concentration has a constant value in the p-Al0.16Ga0.84N electron overflow suppression layer 309 having the largest band gap energy in the laser structure, time lags due to the memory effect will occur at the start and end of doping, thus resulting in a low steepness of the Mg doping profile. Consequently, the p-Al0.16Ga0.84N electron overflow suppression layer 309 will be doped only to an Mg concentration which is about 50% to 70% of the Mg concentration in the cladding layer, and, as such, the absolute amount of Mg is not sufficient. Therefore, the efficiency of hole injection to the active layer is lowered, thus making it difficult to realize an adequate low-threshold current driving with a good reproducibility and uniformity. Moreover, since the pn junction is shifted from the position of the active layer, an increase in the threshold voltage will occur.
In order to increase the Mg concentration in the p-Al0.16Ga0.84N electron overflow suppression layer 309, the p-GaN-acceptor impurity doping start layer 308 may be made thick enough to ensure that the p-Al0.16Ga0.84N electron overflow suppression layer 309 has an Mg concentration which is about the same as the Mg concentration in the cladding layer. However, since a large amount of Mg will be present also in the p-GaN-acceptor impurity doping start layer 308, this Mg will easily diffuse toward the active layer due to a current applied during laser operation, or a heat or magnetic field that is applied to the laser. As a result, the Mg will reach the neighborhood of the active layer, thus causing light absorption losses near the active layer and unfavorably affecting the reliability of the laser. Thus, it has been difficult to realize a highly reliable laser device with a good reproducibility and uniformity.
The present invention has been made in view of the above circumstances, and is intended to provide a highly reliable nitride semiconductor device with a good production yield.